The present invention generally relates to materials and processes for producing directionally-solidified castings, and particularly to a process and apparatus capable of reducing defects in single-crystal (SX) castings, including but not limited to cast components of gas turbines and other high temperature applications.
Components of gas turbines, such as buckets (blades), nozzles (vanes) and combustor components, are typically formed of nickel, cobalt or iron-base superalloys characterized by desirable mechanical properties at turbine operating temperatures. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is an ongoing effort to develop components, and particularly turbine buckets, nozzles, and combustor components, that are capable of withstanding higher temperatures. As the material requirements for gas turbine components have increased, various processing methods and alloying constituents have been used to enhance the mechanical, physical and environmental properties of components formed from superalloys. For example, buckets, nozzles and other components employed in demanding applications are often cast by unidirectional casting techniques to have directionally-solidified (DS) or single-crystal (SX) microstructures, characterized by an optimized crystal orientation along the crystal growth direction to produce columnar polycrystalline or single-crystal articles.
As known in the art, directional casting techniques for producing DS and SX castings generally entail pouring a melt of the desired alloy into an investment mold held at a temperature above the liquidus temperature of the alloy. One such process is represented in FIGS. 1 and 2 as an apparatus 10 that employs a Bridgman-type furnace to create a heating zone 26 surrounding a shell mold 12, and a cooling zone 42 beneath the mold 12. The zones 26 and 42 may be referred to as “hot” and “cold” zones, respectively, which denote their temperatures relative to the melting temperature of the alloy being solidified. The mold 12 has an internal cavity 14 corresponding to the desired shape of a casting 32 (FIG. 2), represented as a turbine bucket. As such, FIG. 1 represents the cavity 14 as having regions 14a, 14b and 14c that are configured to form, respectively, an airfoil portion 34, shank 36, and dovetail 38 (FIG. 2) of the casting 32. The cavity 14 may also contain cores (not shown) for the purpose of forming internal structures such as cooling passages within the casting 32.
The mold 12 is shown secured to a chill plate 24 and initially placed in the heating zone 26 (Bridgman furnace). The heating zone 26 heats the mold 12 to a temperature above the liquidus temperature of the alloy. The cooling zone 42 is directly beneath the heating zone 26, and operates to cool the mold 12 and the molten alloy 16 within by conduction, convection and/or radiation techniques. For example, the cooling zone 42 may be a tank containing a liquid cooling bath 46, such as a molten metal, or a radiation cooling tank that may be evacuated or contain a gas at ambient or cooled temperature. The cooling zone 42 may also employ gas impingement cooling or a fluidized bed.
An insulation zone 44 defined by a baffle, heat shield or other suitable means is between and separates the heating and cooling zones 26 and 42. The insulation zone 44 serves as a barrier to thermal radiation emitted by the heating zone 26, thereby promoting a steep axial thermal gradient between the mold 12 and the cooling bath 46. The insulation zone 44 has a variable-sized opening 48 that, as represented in FIG. 1, enables the insulation zone 44 to fit closely around the shape of the mold 12 as it is withdrawn from the heating zone 26, through the insulation zone 44, and into the liquid cooling bath 46.
Casting processes of the type represented in FIGS. 1 and 2 are typically carried out in a vacuum or an inert atmosphere. After the mold 12 is preheated to a temperature above the liquidus temperature of the alloy being cast, molten alloy 16 is poured into the mold 12 and the unidirectional solidification process is initiated by withdrawing the base of the mold 12 and chill plate 24 downwardly at a fixed withdrawal rate into the cooling zone 42, until the mold 12 is entirely within the cooling zone 42 as represented in FIG. 2. The insulation zone 44 is required to maintain the high thermal gradient at the solidification front to prevent nucleation of new grains during the directional solidification processes. The temperature of the chill plate 24 is preferably maintained at or near the temperature of the cooling zone 42, such that dendritic growth begins at the lower end of the mold 12 and the solidification front travels upward through the mold 12.
FIGS. 1 and 2 represent a single-crystal seed 28 within a cavity 50 at the bottom of the mold 12. The casting 32 epitaxially grows from the seed 28, such that both the primary and secondary crystal orientations are controlled to yield a single-crystal casting. The seed 28 represented in FIGS. 1 and 2 has a cylindrical shape, which is conventional for directional casting techniques, though other shapes are known. FIGS. 1 and 2 further represent a crystal selector 30 coupling the seed cavity 50 to the mold cavity 14, which ensures that a single crystal enters the cavity 14. A bridge 40 connects protruding sections of the casting 32 with lower sections of the casting 32 so that crystal nucleation at these protruding locations can be suppressed and a unidirectional columnar single crystal forms substantially throughout the casting 32.
Mechanical properties of DS and SX castings depend, to a large degree, on the avoidance of grain misorientation defects, for example, high-angle grain boundaries, equiaxed grains, and other potential defects that may occur as a result of the directional solidification process. The avoidance of such defects in a SX casting depends primarily on whether the crystal orientation of the seed 28 can be successfully extended into the casting 32. For this purpose, the seed 28 must be properly oriented at the bottom of the mold 12. In an ideal situation, when the molten alloy 16 is poured into the mold 12 and makes contact with the seed 28, a portion of the single-crystal seed 28 is re-melted. Then, as the mold 12 is slowly withdrawn from the hot zone 26, continuous epitaxial grain growth occurs to yield a single crystal article with an orientation dictated by the single-crystal seed 28.
Although casting processes of the type represented in FIGS. 1 and 2 are typically carried out in vacuum, a thin oxide film can form at the interface between the molten alloy 16 and the single-crystal seed 28 if the alloy and/or seed 28 contains elements capable of chemically reacting with residual oxygen in the vacuum chamber. It is understood that this oxide film is ceramic in nature and can prohibit continuous grain growth from the seed 28, generate misoriented grains, and cause defects in the final casting 32. The formation of an oxide film at the seed-alloy interface can be inhibited by reducing the availability of oxygen and reactive elements within the alloy. However, most nickel-base superalloys used to form single-crystal castings rely on the presence of aluminum to form Ni3Al (gamma prime) as the primary strengthening phase for alloys used to form articles subjected to high stresses in high temperature environments. For example, René N5 (U.S. Pat. No. 6,074,602) contains about 5 to about 7 weight percent aluminum, and CMSX-10 has a nominal aluminum content of about 5.7 weight percent. The oxide films that form during directional solidification of these alloys have been found to typically be aluminum oxide (Al2O3) mixed with chromium oxide (Cr2O3), nickel monoxide (NiO) and titanium oxide (Ti2O3). Due to the high reactivity of aluminum with oxygen, Al2O3 can form at partial pressures of oxygen being as low as 10−18 at a pouring temperature of about 1500° C., which is equivalent to a vacuum of 10−6 torr if water is assumed to be the only residual gas. However, the vacuum in a Bridgman system is typically not better than 10−3 torr. Furthermore, oxygen may be present as an impurity in the molten alloy 16 and/or seed 28, can be present in the mold 12, and can also form as a result of reactions between the mold 12 and the molten alloy 16.
Aside from excluding aluminum from the alloy being cast, attempts to inhibit the formation of an oxide film at the seed-alloy interface have included excluding aluminum from the seed, as reported in U.S. Pat. No. 6,740,176 and U.S. Published Patent Application No. 2010/0058977. However, if aluminum is a required constituent of the seed and/or the casting alloy, it is very difficult to prevent the formation of an oxide film at the seed-alloy interface.